1. Field of the Invention
The invention relates to a microwave window for the spatial, pressure-impervious and diffusion-impervious separation and microwave connection of a first space from/to a second space, with a barrier that has two opposite sides and is at least partially permeable to microwaves. Further, the invention relates to a level-measuring system which works according to the radar principle and has the following: at least one electronic component which generates electromagnetic radiation; at least one antenna which radiates electromagnetic radiation and which receives at least one electromagnetic radiation; and at least one microwave window.
2. Description of Related Art
In industrial metrology, radar-based level-measuring devices are often used to determine the levels of media, such as liquids, bulk material, and even slurries, inside of containers, such as tanks or silos. The transit-time method that is implemented by the measuring devices thereby builds on the physical law that states that the travel path (for example, of an electromagnetic signal) is equal to the product of the duration multiplied by the rate of propagation. In the case of measuring the level of a medium—for example, a liquid or a bulk material—in a container, the travel path is equal to twice the distance between an antenna which radiates and then receives back the electromagnetic signal and the surface of the medium. The wanted echo signal, i.e., the signal which is reflected by the surface of the medium and its duration are determined according to the so-called echo function or the digitized envelope curve. The envelope curve represents the amplitudes of the echo signals as a function of the distance: “antenna—surface of the medium”. A value of the fill level can subsequently be calculated from the difference between the known distance between the antenna and the bottom of the container and the distance between the surface of the medium and the antenna (which has been determined by the measurement). The sent and received electromagnetic signals are most often microwave radiation.
Depending on the nature of the medium or the prevailing process conditions, the following can prevail inside the containers: low or very high pressures and very low or very high temperatures; furthermore, the media can be very aggressive and/or corrosive. In most cases, it is also necessary for the containers to be sealed so that the media cannot escape into the environment. In order to satisfy such general safety conditions, and especially also for the safety conditions concerning the optionally sensitive components of the measuring devices (for example, the temperature sensitivity of the electronic components), there exist, in the state of the art, the microwave windows that were referred to in the beginning. In the presence of high pressures, such windows are preferably composed of a glass or a ceramic, for example, meta-glass, quartz glass, or borosilicate glass; or, if the necessary compressive strength is reduced, of plastics, such as polypropylene, polytetrafluoroethylene (PTFE), or polyetheretherketone (PEEK).
German Patent DE 195 42 525 C2 and corresponding U.S. Pat. No. 5,770,990 describe a microwave window that is arranged inside of a hollow waveguide. Such hollow waveguides, in general, serve to transfer the electromagnetic signals between one of the electronic components that generate the signals and process the received signals, and the antenna which radiates and receives the electromagnetic signals. The microwave window effects, on the one hand, a pressure-impervious and diffusion-impervious separation between the internal space of the container and the external space. and on the other hand, a transfer of microwaves between the spaces.
Electromagnetic radiation is produced, in general, by the electronic components in the form of spherical waves, which are converted into planar waves for level measurement, and therefore, for emission into the container in which the medium whose level is to be determined resides. The waves, which are reflected by the surface of the medium, are converted back into spherical waves. For this, the prior art provides for the use of lenses (see, for example, International Patent Application Publication WO 2011/110560 A1). In German Patent Application DE 44 07 823 A1, a configuration is described in which lamellae are arranged as an inhomogeneous lens on a plate that separates the antenna from the process space.